The present invention relates to a developing apparatus capable of recording images in two or more colors and, more particularly, to a developing apparatus having a plurality of developing units, each for forming a thin one-component toner layer on the surface of a developing sleeve, which can be set in a multi-color recording mode and a single-color recording mode.
In order to record images in two or more colors on a photosensitive body by means of a developing apparatus of this type, different bias voltages are applied to the developing sleeves of the developing units.
That is, to record an image in the first color, a developing bias voltage, obtained by superposing an AC bias voltage on a DC bias voltage, is applied to a developing sleeve of a first developing unit. To record the same image in the second color on the first-color image, however, a DC bias voltage is applied to the developing sleeve of a second developing unit for the second color so that toner particles of the first color on the photosensitive body do not fly to the developing unit of the second color.
When the DC developing bias voltage is applied to the developing sleeve, developing efficiency, i.e., developing agent flying efficiency per unit area of the developing sleeve is lower than in the case where the developing bias voltage, obtained by superposing the AC developing bias voltage on the DC developing bias voltage, is applied to the developing sleeve. For this reason, less developing agent needs to be supplied to the photosensitive body during development of the second than during development of the first color. Therefore, the second-color image is inevitably less dense than the first-color image, making it difficult to obtain a good image.